Sanitary protection products, such as sanitary napkins, panty liners, incontinence products, diapers and bedding underpads, are typically comprised of a body facing, liquid permeable cover, a garment facing, substantially liquid impermeable barrier sheet and an absorbent structure therebetween. The liquid impermeable barrier sheet is typically made of a thin, flexible plastic film that is impermeable to both liquids and vapors. The liquid permeable cover is also quite often made of a plastic film, similar to that of the barrier sheet, that is made perforate by creating two or three dimensional perforations in the film, thereby leaving plastic film land areas between the perforations. Such barrier sheets, as well as the land areas of the liquid permeable plastic film covers, do not permit vapors of liquids absorbed in the product to pass out from the product or permit liquids that collect on the surface of the user's body to enter into the product, the liquids being such as menstrual fluids, urine and perspiration. Such products typically feel uncomfortably hot when dry and clammy when wet.
Prior inventors have attempted to facilitate the transmission and removal of vapors from absorbent products by using inherently vapor transmitting liquid barrier materials or by creating, in liquid barrier materials, pores that are large enough to permit passage of vapors but not the passage of liquids. Such materials, and the products made therefrom, are commonly described as "breathable". Vapor permeable pores may be grouped into two categories, micropores and macropores, these being contained in microporous and macroporous materials respectively. Microporous materials are most resistant to liquid penetration and exhibit Frazier air permeability values of zero mm.sup.3 /m.sup.2 /min. However, they are also most resistant to vapor permeation, and therefore likely to be perceived by the user as not being breathable and not providing comfort and a dry feeling during wear. Macroporous materials, on the other hand, are most likely to be perceived as being breathable, exhibiting Frazier air permeability values that are greater than zero mm.sup.3 /m.sup.2 /min, thereby providing such comfort and dry feeling; but are also most likely to permit liquids to leak and therefore not provide protection against leakage from the absorbent product onto the user, the user's garment and beddothes.
Microporous Structures
Microporous structures are for the most part films with effective vapor transmitting micropore sizes that are equal to or greater than 100 Angstroms. Films may be inherently microporous, as for example those films made of polyurethanes. Such a film formed onto a base woven or nonwoven fabric is described in U.S. Pat. No. 4,560,611, where the coating solution consists of a polar organic solvent containing a polyurethane elastomer, a water repellent agent, e.g., a fluorine or silicone based material, a polyisocyanate and a nonionic surfactant. U.S. Pat. No. 4,197,371 discloses a sheet material of natural or synthetic rubber or a rubberlike polymer having uniformly incorporated particles of at least one swellable modified polymer such as modified starches and celluloses. U.S. Pat. No. 4,178,271 discloses a similar sheetlike material where the film is polyvinyl chloride or its copolymer. U.S. Pat. No. 3,869,310 discloses a leatherlike flexible sheet material, comprising a nonwoven fibrous mat and a polymeric impregnant, that has a porous structure and is not bonded to the fibers of the mat. The mat, composed of fibers prepared from at least two different polymeric materials, is first impregnated with a first liquid, that is a solvent for one of the polymeric materials and a nonsolvent for the other, to dissolve the soluble fibers; and then adding a second liquid, that is partially miscible with the first liquid and is a nonsolvent for all the polymeric fiber materials, to coagulate the resulting polymer solution.
Pores may be created in inherently nonporous films by means such as: stretching films in which thinned or stressed regions have been created or which noncompatible (to the film) inclusions have been incorporated. The stretching cause microfissures to form in the thinned or stressed regions or microseparations to form between the film and the noncompatible inclusions. Other means to create micropores comprise the incorporation in a film of soluble or volatile indusions that are removed by dissolving or volatilizing such inclusions. Still other means provide the blending into a polymer of fragmentable or abradable particles to form a sheet and then subjecting the sheet to a compressive force that breaks the particles to form micropores or to abrade the sheet to form micropores. UK Patent Application GB No 2,026,381 discloses the preparation of porous membranes by blending a polymer with a liquid component to form a binary two-phase system which in the liquid aggregate state has regions that are miscible and regions that have miscibility gaps. UK Patent Application GB No 2,115,702B discloses a liquid impermeable, vapor permeable backing that is composed of a film made by molding a mixture of a polyolefin resin and a liquid or waxlike hydrocarbon polymer into a film and then stretching the film laterally and/or longitudinally to more than 1.2 times its original dimension to create fine pores in the film. U.S. Pat. Nos. 3,953,566, 3,962,153, 4,096,227, 4,110,392, 4,187,390 and 4,194,041 disclose the preparation of porous sheets, and other porous articles, by extruding a paste comprised of particles of polytetrafluoroethylene, which is a nonthermoplastic polymer, and a lubricant, and then removing the lubricant and stretching and annealing the resultant product. The resulting product is a sintered, oriented porous film characterized by having polymer nodes connected by fibrils. Somewhat related to these patents, and yielding a soft dothlike liquid permeable sheet material, is U.S. Pat. No. 4,622,036 which discloses such a sheet material consisting of particles of nondissolvable polyolefin or polyvinyl chloride that are partially fused together by heat so as to provide a desired amount of liquid permeability, the particles ranging in size from about one to 2000 microns, and the sheet having a thickness from about 0.0005 to 250 inches.
U.S. Pat. Nos. 4,100,238 and 4,197,148 describe the preparation of microporous films by extruding a two component blend from which one component is leached out with a solvent, and then stretching the leached film to obtain a desired porosity in a soft dothlike liquid permeable sheet material. U.S. Pat. Nos. 3,214,501, 3,640,829 and 3,870,593 disclose the preparation of a microporous polymer sheet by blending into a polymer nonmiscible, nonleachable fillers such as starch and salts, forming the sheet and then stretching the sheet to form pores that are initiated at the sites of the filler particles. U.S. Pat. No. 4,347,844 discloses the preparation of a porous sheet, for use in a disposable diaper, by blending a particulate substance into a polymer, forming a sheet and then breaking the particulate substance within the sheet under a compressive force to create micropores. U.S. Pat. No. 4,308,303 discloses a flocked foam coated fiber reinforced water vapor permeable barrier, having a fabric appearance and capable of filtering bacteria, comprising a microporous polyolefin film coated on at least one surface with a foamed latex polymer, flocked fibers on the outside of the foamed latex polymer and a web of spunbonded fibers on the outside of the flocked foamed latex polymer. The film is rendered microporous by stretching. The film becomes microporous because it has minute fracture sites or pore nucleating agents such as finely divided filler, preferably calcium carbonate, of particle size less than 3 microns, and/or minute crystalline domains. U.S. Pat. No. 4,609,584 discloses the preparation of a porous sheet, for use in a disposable diaper, by blending a particulate substance into a polymer, forming a sheet and then abrading or buffing the surface of the sheet to create micropores. U.S. Pat. Nos. 4,539,256 and 4,609,584 disclose methods for making microporous sheets that comprise the steps of melt blending a crystallizable thermoplastic polymer with a compound that is miscible with the polymer at the polymer's melting temperature, but not below that temperature, forming a sheet of the melt blend, and then cooling the sheet to a temperature at which the compound becomes immiscible with the polymer and phase separates. When the sheet is oriented at least in one direction, a network of interconnected micropores forms between the polymer phase and the compound phase. The compound may be removed from the sheet by solvent extraction.
U.S. Pat. No. 3,156,242 discloses a flexible absorbent sheet, of for example polyethylene, that is useful as a backing sheet or as an outer layer of a surgical dressing, the sheet being microporous. However, the sheet may have holes or slits formed in it, to make it macroporous. U.S. Pat. No. 3,426,242 discloses a breathable medical dressing having a backing comprised of an open celled structure, i.e., a film processed to have voids with passageways to its outside surfaces that are generally less than 5000 Angstroms, e.g., from 100 to 5000 Angstroms, the film having a final crystallinity of at least 40%. The film is preferably coated with a continuous layer of microporous pressure sensitive adhesive. U.S. Pat. No. 3,932,682 describes waterproof products, capable of transmitting air and water vapor, that are made by spray spinning filamentary material, e.g., by meltblowing, onto an open celled polymer film to obtain thermal selfbonding or by spraying the filamentary material onto an elastic film, then stretching the resulting product to yield an open celled structure and heat setting the stretched product to impart dimensional stability. U.S. Pat. Nos. 4,758,239 and 4,818,600 disclose a breathable barrier that includes a fibrous porous sheet, preferably a meltblown web, to which is joined a nonmicroporous film, wherein some of the fibers at the joined surface are intimately commingled with the film to give a vapor transmission rate at 37.degree. C. and 50% RH of about 100-5000 g/m.sup.2 /24 hrs and is impermeable to 0.9% saline for at least one hour at 21.degree. C. for about one hour at a hydrostatic head of at least 11.4 cm. The film may be a preformed film of a water soluble polymer such as polyvinyl alcohol.
Macroporous Structures
U.S. Pat. No. 3,989,867 discloses an absorptive device having a backsheet with bosses and uniformly small apertures at the apices of the bosses. The apertures take up from 0.5 to 10% of the available permeation area of the backsheet to allow vapor transmission while preventing liquid passage at pressures typically encountered in use. U.S. Pat. No. 4,591,523 describes an apertured, macroscopically expanded, three dimensional polymeric web useful as a breathable, fluid resistant barrier for a disposable diaper. The web preferably comprises a deeply drawn three dimensional structure containing a multiplicity of debossments of macroscopic cross section. Each debossment originates as an aperture in a first surface of the web, has continuous sidewalls that terminate to form an end wall in a second parallel surface of the web, the end wall including multiple apertures that are sized and shaped to support an aqueous fluid meniscus and being spaced one from another so that the fluid menisci do not contact each other. U.S. Pat. No. 4,059,114 describes a disposable shield for garment protection and every day feminine hygiene that has a fluid barrier in the form of a soft, rattle free moisture permeable layer that is preferably a liquid impermeable layer of a blown microfiber web. U.S. Pat. No. 5,591,510 discloses a layered fabric material that is breathable and resists fluid penetration which comprises a breathable and fluid penetration resistant nonwoven and a plastic film that has perforations that are disposed from about 5.degree. to 60.degree. to the film's surface.
It is an objective of this invention to provide perceptibly breathable vapor permeable, macroporous barrier structures, and sanitary protection products made therefrom, that restrict the passage of liquids under use conditions, and therefore give protection against liquid leakage as well as user-perceptible comfort and dry feeling.